Communication technologies and uses have greatly changed over the last few decades. In the fairly recent past, copper wire technologies were the primary mechanism used for transmitting voice communications over long distances. One alternative that has grown in recent years is the use of cellular or mobile phones and systems. These systems allow personal telephone communications over different wireless mediums which greatly increase the ability of people to communicate and/or transfer data wherever they may be. One outcome of these wireless based communication systems, is the possibility to determine the position of a mobile device which has enabled application developers and wireless network operators to provide location based, and location aware, services. Examples of these services are guiding systems, shopping assistance, friend finder and other information services giving the mobile user information about their surroundings.
In addition to the commercial services, the governments in several countries have also put requirements on the network operators to determine the position of an emergency call. This has led to the ability for network operators to determine the position of a mobile device that is currently turned on, e.g., in an idle or busy mode, when requested by an appropriate authority. Two general methods of positioning can be described as either external methods, e.g., Global Positioning System (GPS) based methods like Assisted-GPS (A-GPS), or methods which use the wireless networks. Methods which use the wireless networks can be further broken down into two main groups. The first group includes methods that are based on the radio cell to which a mobile terminal is attached, e.g., by the using cell ID of the cell to which the mobile terminal is currently assigned. The second group uses measuring of radio signals from several base stations and determining the terminal position using, e.g., Time Difference (TD).
In order to be able to connect to a mobile network or to perform handover when connected, a mobile terminal is typically constantly measuring available signals, not only from their own base station, but also from other base stations. These signals are typically control signals intended for measuring radio conditions of transmission, which control signals contain, among other data, information about how to establish a connection to the transmitting base station. In particular, the control signals include data, which by itself or in combination with the frequency of the carrier on which the control signal was transmitted, constitute base station identification data. A mobile terminal can thus obtain an identity of the transmitting base station and an estimate of the radio conditions. The mobile terminal typically compiles this information, e.g., in Global System for Mobile communications (GSM) systems, in a neighbor list, which is transferred to the network as information.
Position estimation can be based on measurements in the neighbor list. For this, one then uses the relation between the distance from the radio base station and the radio condition in combination with knowledge about the exact position of the base station. The base station positions are known within the communications network. This means that the neighbor list can be used for position estimation according to different algorithms. The accuracy of the position estimation is generally proportional to the size of the cell.
Triangulations or TD methods, use signals associated with two or more different base stations. These signals are used to calculate the position of a mobile terminal or the distance from the base station at which a mobile terminal is located. The calculations are based on the relative or absolute difference in the time that it takes for a signal to propagate between the terminal and the different base stations. The achievable accuracy of TD methods depends, for example, on system architecture, physical conditions and radio conditions. Typically, the accuracy of a TD method for a mobile telephony system is 50 to 150 meters, however, TD methods are relatively time and resource consuming.
In the current market for mobile positioning systems, the most popular and standardized mobile positioning methods are Cell ID based, A-GPS and Uplink Time Difference of Arrival (UTDOA). Both A-GPS and UTDOA require either relatively significant hardware and/or software modification to be implemented in a wireless system. For example, A-GPS requires special chipsets to be present in the mobile device and UTDOA requires hardware, e.g., local measurement units, to be installed at multiple base stations to perform accurately. These hardware requirements make both A-GPS and UTDOA alternatives relatively more expensive than mobile positioning methods which are Cell ID based, since Cell ID based methods tend to require no additional hardware or special software changes to a mobile terminal. Additionally, Cell ID based mobile positioning methods can act as an effective backup when other positioning methods fail or are not available.
In its simplest form, the Cell ID method of position can be conceptualized as shown in FIG. 1. Therein, a mobile station or terminal MS 10 which is connected to a base station 12 is known to be somewhere within the region 13 defined by that base station 12's 120 degree directional antenna (not shown) and outer periphery 16 associated with the base station 12's transmission/reception range. Thus the MS 10's position can be reported to be that of a central point 18 within that sector, the accuracy of which will be determined by the size of the cell and the MS's actual position relative to point 18.
An enhancement to the basic Cell ID positioning technique involves using timing advance (TA) information to improve the granularity of positioning. TA information is available in the network for each connection between an MS 10 and a BS 12 and relates to the amount of time for a signal transmitted by MS 10 to reach BS 12. Initially used for adjusting uplink transmission times to control interference, the TA information also provides a rough estimate of a connected MS's relative distance to its BS 12. For positioning purposes, this means that in addition to the Cell ID, an MS 10's position can be further pinpointed to within an arcuate band associated with its current TA as shown in FIG. 2. Although only five TA bands are shown in FIG. 2, it will be appreciated that most radio communication systems will have more TA values.
More recently, another modified Cell ID based global positioning method known as Enhanced Cell Global Identity (ECGI) has been introduced, which method uses information from a mobile system measurement report in addition to the timing advance (TA) value to improve the accuracy of currently used CGI/TA methods. ECGI is described, for example, in U.S. Patent Publication No. 2006/0267840, the disclosure of which is incorporated here by reference. Among other things, the ECGI algorithm uses the difference in path loss of the serving and the co-sited neighbor cells in combination with the TA value and serving cell coordinates in order to estimate the mobile system position.
However, as will be described below, the use of pathloss measurements introduces other potential inaccuracies into the output of, e.g., the ECGI algorithm. Accordingly, it would be desirable to provide devices, systems and methods for improving the accuracy of positioning methods.